In the biological treatment of water, microorganisms derive energy from an electron donor by facilitating the transfer of an electron to an electron acceptor. As used herein, an electron donor is a source of food that is often comprised of a carbon source and an electron acceptor is often dissolved oxygen, but can also be nitrate, sulfate, perchlorate, or many other compounds. During groundwater remediation, the perchlorate ion is of concern due to its potential human health effects. It is desirable to remove perchlorate from groundwater and drinking water. It is also of benefit to remove perchlorate from water without significantly increasing the biological oxygen demand (BOD) of the water.
In the field of septic wastewater treatment, there has been growing concern about the release of nitrate into the environment due to its linkage to methaemoglobinaemia, a condition where most typically babies do not utilize oxygen properly. Septic wastewater will often contain high BOD due primarily to biodegradable carbon. Septic wastewater will also often contain high nitrogen, including reduced and oxidized forms. To treat this wastewater, aerobic biological treatment is often performed, where oxygen is added to the water and microorganisms consume the carbon to decrease the BOD. Through this aerobic process, reduced forms of nitrogen, such as in ammonia or ammonium, will be transformed to oxidized states, with a typical endpoint of nitrate; this process is called nitrification.
After nitrification, the nitrate requires removal. Nitrate acts as an electron acceptor, but most of the electron donor (the carbon) has been removed during the aerobic treatment phase. Thus, an external carbon source may be required to reduce the nitrate. During this denitrification step, the nitrate is converted to nitrogen gas, which diffuses out of the water, thus removing nitrogen from the water; however, excess carbon may remain, which can lead to a high BOD value, which is normally considered itself to be a pollutant.
Another approach typically used to reduce nitrate is to use the carbon that is already present in the wastewater as the carbon source. This approach normally involves two treatment zones: The first treatment zone is a denitrification zone where carbon is consumed; the second treatment zone is a nitrification zone where oxygen is added and ammonia/ammonium is converted to nitrate. See, for example, U.S. Pat. No. 5,676,828 (an ammonification and denitrification reactor is coupled with a nitrification reactor, where ammonification is the process of creating ammonia/ammonium, and the final effluent comes from the ammonification and denitrification reactor). The effluent from the second or nitrification treatment zone is recycled, or looped back, to the first or denitrification zone such that the denitrification zone is receiving both nitrified water and raw water containing the carbon. A portion of either the nitrification or denitrification zones can be discharged as effluent. This treatment method is advantageous because an external carbon source is not required; however, this treatment method is not capable of achieving very low levels of both nitrogen and carbon because the final effluent is either coming from the denitrification zone—which contains ammonia/ammonium that may be later converted to nitrate in the environment—or the final effluent comes from the nitrification zone—which contains some nitrate.
Another approach adds an external carbon source. This approach involves the excavation of soil by digging a substantial trench in a target area to a depth below the water table for that location, filling that trench with a body of organic carbon such as wood chips, and thereafter covering the excavated area with layered porous material such as sand or gravel. See, for example, U.S. Pat. No. 5,318,699 (Robertson et al.) (construction of a soak away structure and installing a large body of organic carbon below a septic tank system). The large body of organic carbon provides both a carbon source and a location for growth of microorganisms required to breakdown the contaminants. Sufficient carbon is provided in the large body to last for several decades or longer; however, one of the problems with this system is that it does not control the amount of carbon available for denitrification. The system may release too much carbon in the effluent, which is itself often considered a pollutant, and is thus an undesirable effect.
In another approach involving an external carbon source a tank is constructed or installed downstream from a reservoir used to collect agricultural run-off. See, for example, U.S. Pat. No. 5,330,651 (Robertson et al.). The tank is filled with a substantial amount of organic carbon such as wood chips or sawdust. This approach requires that the contaminated water must remain in contact with treatment material in the tank for a substantial residence time to ensure breakdown of the contaminants. Again, there is the potential for an excess of carbon to be released.
In another approach phospohorus, carbon, and nitrogen are treated in two primary modes. See, for example, U.S. Pat. No. 5,342,522 (Marsman et al.). The first mode is plug flow where carbon and phosphorus removal occurs in a first step, followed by nitrification in a second step, and denitrification using an added carbon source is used in a third step. The added carbon may be from the sludge of the first step or may be an external carbon source. This approach requires perfect matching of added carbon and nitrate, which is difficult to perform, and may lead to significant levels of either carbon or nitrate in the effluent. In the second mode denitrification is followed by a nitrification step used in combination with recycling between stages; however, this is subject to the limitations previously discussed regarding trying to achieve very low levels of nitrogen.
Another approach uses the measurement of oxidation reduction potential (ORP) to optimize the addition of external carbon (for example, methanol) for the removal of nitrate. See, for example, U.S. Pat. No. 5,556,536 (Turk) and U.S. Pat. No. 5,482,630 (Lee et al.). The goal of this approach is similar to the goal of the present invention—to achieve very low levels of carbon and nitrate; however, this method may require careful monitoring to achieve very low levels of both nitrate and carbon.
Further approaches include U.S. Pat. No. 5,520,812 (Ryhiner et al.), who describes a two-region system with recycling and adds partial oxygenation of the denitrification region; U.S. Pat. No. 5,288,407 (Bodwell et al.), who describes a system involving two tricking filters with recycling to reduce carbon and to nitrify and a separate denitrification media for the denitrification portion, suggesting the use of a sulfur-limestone media; and U.S. Pat. No. 4,374,730 (Braha et al.); U.S. Pat. No. 4,179,374 (Savage et al.); and U.S. Pat. No. 4,160,724 (Laughton), who involve some form of recycling between denitrification and nitrification zones.
What would thus be desirable is provide a water treatment method and apparatus for practically complete removal of electron acceptors (excluding oxygen) from waters of various origins, while ensuring that an added excess of external carbon source is also practically completely removed. It would be further desirable to provide a water treatment method and apparatus that allows for a margin of error in setting the carbon addition rate. It would be further desirable to provide a water treatment method and apparatus capable of achieving very low levels of the target electron acceptor(s) and constituents that are potentially convertible to the target electron acceptor(s), and an added external carbon source.